The may also have helped defend against predators, including carnivorous marsupials, saber-toothed cats, and so-called terror birds, which grew up to ten feet (three meters) tall (photo of a terror-bird model), he said.

Larger mammals also have the advantage of access to low-quality food resources, such as wood, that smaller species are unable to digest, the researcher added.

"There are many advantages for a big mammal," he said. "Some of these reasons could be the explanation" for J. monesi's size.

The fossil discovery takes the title of rodent heavyweight champion away from another extinct South American species, Phoberomys pattersoni. Likened to a giant guinea pig, P. pattersoni weighed an estimated 1,500 pounds (700 kilograms). (See pictures of the prehistoric "giant guinea pig".)

Of the latest find, Sánchez-Villagra said, "This undoubtedly is the largest rodent that we know of. It's quite remarkable. But I'm not envious—I think it's great.

"I suspected there would be larger rodents out there," he said. "There are probably others that are bigger still."

The latest find is further evidence of the incredible diversity seen in South American fossil rodents, Sánchez-Villagra said.

"There were many species with no equivalent today, especially in terms of size," he added.

The likely reason for such variety is that South America's rodents evolved in isolation, the paleontologist said.

Rodent Island

"For most of the last 65 million years, since shortly after the dinosaurs became extinct, South America was an island," he said.

But around three million years ago the Panama land bridge formed, linking North and South America. New types of mammals flooded in from the north, perhaps sending bizarre species such as J. monesi scurrying to extinction.

Newcomers included predatory cats and bears that were previously absent in South America, Sánchez-Villagra said.

"Large herbivores from North America probably competed with the big rodents," he added.

Climate change likely also contributed to the demise of massive rodents, the paleontologist said.

The new discovery should provide important new clues to the growth processes that produced such massive rodents, he said.

Rinderknecht, the study author, said the team is now investigating the fossil skull to work out the animal's hearing and smelling capabilities.